[Linked Image from i.imgur.com]
You can just pour dry Quikrete into the hole and it will set. I have done them that way and it works.
But if you want it to last a long time you don't want that wood underground. Yes, there are guys who have posts that have been underground for 30 years and they are OK.
Thirty years ago they put arsenic in the PT wood. EPA took the arsenic out 15 years ago.

You need to buy a galvanized steel "chair" at Lowes. You buy a 1/2 inch galvanized steel bolt, it has an "L" shape at the bottom and is 8 inches long.
Set that into the wet concrete. Come back 2 days later and bolt the chair onto the bolt.

One little mistake I made here. Make a 2x4 frame 14x14 inches and set it on the hole. Pour the concrete it will then be 3 1/2 inches above the ground. Will keep dirt off of the chair and the post.